A light emitting diode (LED) is an electroluminescent device having a structure in which an N-type semiconductor and a P-type semiconductor are joined together, and emits light through recombination of electrons and holes. Such an LED has been widely used for a display and a backlight. Further, since the LED has less electric power consumption and a long life span as compared with conventional electric bulbs or fluorescent lamps, its application area has been expanded to the use thereof for general illumination while substituting for conventional incandescent bulbs and fluorescent lamps.
The LED repeats on/off in accordance with the direction of a current under an AC power source. Thus, if the LED is used while being connected directly to the AC power source, there is a problem in that it does not continuously emit light and is easily broken by means of a reverse current.
To solve such a problem of the LED, an LED that can be used while being connected directly to a high voltage AC power source is proposed in International Publication No. WO 2004/023568A1 entitled “LIGHT-EMITTING DEVICE HAVING LIGHT-EMITTING ELEMENTS” by SAKAI et al.
According to the disclosure of WO 2004/023568A1, LEDs are two-dimensionally connected in series on an insulating substrate such as a sapphire substrate to form LED arrays. Two LED arrays are connected in reverse parallel on the sapphire substrate. As a result, there is provided a single chip light emitting device that can be driven by means of an AC power supply.
However, since the sapphire substrate has relatively lower thermal conductivity, heat cannot be smoothly dissipated. This limitation on the heat dissipation leads to a limitation on the maximum luminous power of the light emitting device. Therefore, there is a need to improve a heat dissipation property in order to increase the maximum luminous power of a light emitting device under a high voltage AC power source.
Further, since the LED arrays are alternately operated under the AC power source in the light emitting device, the luminous power is considerably limited as compared with a case where light emitting cells are simultaneously operated. As a result, in order to increase the maximum luminous power, it is necessary to improve the light extraction efficiency of each light emitting cell.